Crustal anatomy of a transform continental margin

Abstract

This paper describes new deep Seismic reflection results across the major transform margin southwest of the Grand Banks, off eastern Canada. An interpretation of these results is presented, supplemented with previous Seismic refraction, gravity and magnetic measurements.The results show a fairly sharp transition between oceanic and continental regions at the transform margin. A narrow zone, about 40 km wide, separates the two regions and is interpreted to be a zone in which shearing has destroyed the original fabric of the continental crust. Faults in this region extend throughout the crust and there is some evidence that a major strike-slip fault may extend some tens of kilometres into the upper mantle. Magmatism may have further modified the crust in this shear zone, which is bounded on the oceanic side by a small seamount. The continental crust is thinned by a factor of 3 or more in this zone, which may be due to erosion or to flow in the lower crust during shearing. The oceanic crust does not appear to thin significantly as the transform margin is approached. Crustal-scale dipping reflectors over a confined region below the shelf are interpreted to represent the contact between two Appalachian terranes: the Meguma and Avalon terranes. This supports earlier interpretations of a large magnetic lineament in the region as marking this boundary.These crustal Seismic data crossing a large transform margin are the first of their kind and will help us to constrain evolutionary models of transform margins. The picture they provide is different in many respects from that across rifted margins and similar in others. In some cases it might be difficult to distinguish between these two classes of Atlantic-type margins on the basis of crustal structure.